Gardiner Hubbard attended Phillips Academy, Andover and later graduated from Dartmouth in 1841. He then studied law at Harvard, and was admitted to the bar in 1843. He later lived in the adjoining city of Cambridge and joined a Boston law firm, practicing his profession in Boston until 1873, when he relocated to Washington, D.C. Gardiner Hubbard helped establish a city water works in Cambridge, was a founder of the Cambridge Gas Co. and later organized a Cambridge to Boston trolley system.

Hubbard married Gertrude Mercer McCurdy (New York City, March 12, 1827 – October 20, 1909, Washington, D.C.) and had six children: Robert Hubbard (1847–1849); Gertrude Hubbard (1849–1886); Mabel Gardiner Hubbard (1859–1923); Roberta Hubbard (1859–1885); Grace Hubbard (1865–1948); and Marian Hubbard (1867–1869).[3] Gardiner Hubbard's daughter Mabel became deaf at the age of five from scarlet fever.[4] She later became a student of Alexander Graham Bell, who taught deaf children, and they eventually married.[5] Hubbard also played a pivotal role in the founding of Clarke School for the Deaf, the first oral school for the deaf in the United States located in Northampton, Massachusetts.

Hubbard argued for the nationalization of the telegraph system (then a monopoly of the Western Union Company, as he explained) under the U.S. Postal Service stating in an article: "The Proposed Changes in the Telegraphic System",[6] "It is not contended that the postal system is free from defects, but that it removes many of the grave evils of the present system, without the introduction of new ones; and that the balance of benefits greatly preponderates in favor of the cheap rates, increased facilities, limited and divided powers of the postal system."

During the late 1860s, Gardiner Hubbard had lobbied Congress to pass the U.S. Postal Telegraph Bill that was known as the Hubbard Bill. The bill would have chartered the U.S. Postal Telegraph Company that would be connected to the U.S. Post Office. The Hubbard bill did not pass.

To benefit from the Hubbard Bill, Hubbard needed patents which dominated essential aspects of telegraph technology such as sending multiple messages simultaneously on a single telegraph wire. This was called the "harmonic telegraph" or acoustic telegraphy. To acquire such patents, Hubbard and his partner Thomas Sanders (whose son was also deaf) financed Alexander Graham Bell's experiments and development of an acoustic telegraph, which serendipitously led to his invention of the telephone.

Hubbard organized the Bell Telephone Company on July 9, 1877, with himself as president, Thomas Sanders as treasurer and Bell as 'Chief Electrician'. Hubbard also became the father-in-law of Bell when his daughter Mabel Hubbard married Bell on July 11, 1877.

Gardiner Hubbard was intimately connected with the Bell Telephone Company, which subsequently evolved into the National Bell Telephone Company and then the American Bell Telephone Company, merging with smaller telephone companies during its growth. The American Bell Telephone Company would, at the very end of 1899, evolve into AT&T, at times the world's largest telephone company.

Hubbard also became a principal investor in the Edison Speaking Phonograph Company. When Edison neglected development of the phonograph, which at its inception was barely functional, Hubbard helped his son-in-law, Alexander Graham Bell, organize a competing company in 1881 that developed wax-coated cardboard cylinders and disks for used on a graphophone. These improvements were invented by Alexander Bell's cousin Chester Bell, a chemist, and Charles Sumner Tainter, an optical instrument maker, at Alexander Graham Bell's Volta Laboratory in Washington, D.C. Hubbard and Chester Bell approached Edison about combining their interests, but Edison refused,[7] resulting in the Volta Laboratory Association merging the shares of their Volta Graphophone Company with the company that later evolved into Columbia Records in 1886.

The main school building at the Clarke School for the Deaf, Hubbard Hall, is named after him in his honor.

Hubbard's house on Brattle Street in Cambridge (on whose lawn, in 1877, Hubbard's daughter Mabel married Alexander Graham Bell) no longer stands. But a large beech tree from its garden still (in 2011) remains. After he moved to Washington, D.C. from Cambridge in 1873, Hubbard subdivided his large Cambridge estate. On Hubbard Park Road and Mercer Circle (Mercer was his wife's maiden name) he built large houses designed for Harvard faculty. On nearby Foster Street, he built smaller houses, still with modern amenities, for "the better class of mechanic." This neighborhood west of Harvard Square in Cambridge is now both popular and expensive. For construction dates of individual houses, see http://hul.harvard.edu/huarc/refshelf/cba/h.html#hubbardpkrd and http://hul.harvard.edu/huarc/refshelf/cba/m.html#mercercir

To service his then-modern Cambridge house, Hubbard wanted gas lights, the then-new form of illumination. So he founded the Cambridge Gas Company, now part of NSTAR.

After his move to Washington, Hubbard helped to found the National Geographic Society and served as its first president. Today, its Hubbard Medal is given for distinction in exploration, discovery, and research. In 1897, he also helped to rescue the A.A.A.S, the American Association for the Advancement of Science (founded in 1848) from financial peril and extinction by enabling its purchase of the (then privately owned) "Science" magazine.[9]

In 1899, a new school on Kenyon Street in Washington, DC was named the Hubbard School in his honor as one of the "most public-spirited men of the District, never neglecting an opportunity to advance its interests, but was also a man of great learning and earnestly interested in all educational movements. Mr. Hubbard was the president of the National Geographic Society, a man prominent in science and a man of the highest character."[10] The school has since been closed and demolished.